Air-water Temperature Difference Research Articles

Cranes soar on thermal updrafts behind cold fronts as they migrate across the sea.

Thermal soaring conditions above the sea have long been assumed absent or too weak for terrestrial migrating birds, forcing obligate soarers to take long detours and avoid sea-crossing, and facultative soarers to cross exclusively by costly flapping flight. Thus, while atmospheric convection does develop at sea and is used by some seabirds, it has been largely ignored in avian migration research. Here, we provide direct evidence for routine thermal soaring over open sea in the common crane, the heaviest facultative soarer known among terrestrial migrating birds. Using high-resolution biologging from 44 cranes tracked across their transcontinental migration over 4 years, we show that soaring performance was no different over sea than over land in mid-latitudes. Sea-soaring occurred predominantly in autumn when large water-air temperature difference followed mid-latitude cyclones. Our findings challenge a fundamental migration research paradigm and suggest that obligate soarers avoid sea-crossing not due to the absence or weakness of thermals but due to their low frequency, for which they cannot compensate with prolonged flapping. Conversely, facultative soarers other than cranes should also be able to use thermals over the sea. Marine cold air outbreaks, imperative to global energy budget and climate, may also be important for bird migration.

A New Approach for Assessing Heat Balance State along a Water Transfer Channel during Winter Periods

Ice problems in channels for water transfer in cold regions seriously affect the capacity and efficiency of water conveyance. Sometimes, ice problems such as ice jams in water transfer channels create risk during winter periods. Recently, water temperature and environmental factors at various cross-sections along the main channel of the middle route of the South-to-North Water Transfer Project in China have been measured. Based on these temperature data, the heat balance state of this water transfer channel has been investigated. A principal component analysis (PCA) method has been used to analyze the complex factors influencing the observed variations of the water temperature, by reducing eigenvector dimension and then extracting the principal component as the input feature. Based on the support vector machine (SVM) theory, a new approach for judging the heat loss or heat gain of flowing water in a channel during winter periods has been developed. The Gaussian radial basis is used as the kernel function in this new approach. Then, parameters have been optimized by means of various methods. Through the supervised machine learning process toward the observed water temperature data, it is found that the air–water temperature difference and thermal conditions are the key factors affecting the heat loss or heat absorption of water body. Results using the proposed method agree well with those of measurements. The changes of water temperature are well predicted using the proposed method together with the state of water heat balance.

Evaporating waterbody effects in a simplified urban neighbourhood: A RANS analysis

The incorporation of nature-based solutions comprising green and blue infrastructure is often touted as a way to cool cities and enhance pollutant removal. However, there is little agreement between different methodologies to measure the effect of any single intervention. Here, we present 3D steady RANS simulations to investigate the influence of waterbody on in-canyon flow structure, temperature (T∗) and water vapour (ω∗) distribution in a simplified urban neighbourhood. A novel solver that captures evaporation effects is developed and validated against wind tunnel experiments. Simulations are performed under neutral atmospheric conditions for forced- and mixed-convection cases and different air–water temperature differences, indicative of either daytime or night-time conditions. Results under forced convection show minimal impact on the flow structure, whilst T∗ and ω∗ effects are distributed primarily over and around the water surface. However, the mixed-convection case shows that a cooler waterbody weakens the principal vortex in the open square, whilst T∗ and ω∗ effects reach further upwind and are more widely distributed in the spanwise direction. A warmer waterbody is shown to disrupt the skimming flow structure, indicating a possible heat and pollutant removal mechanism from around the waterbody and also downwind canyons.

Laboratory Heat Flux Estimates of Seawater Foam for Low Wind Speeds

Laboratory experiments were conducted to measure the heat flux from seafoam continuously generated in natural seawater. Using a control volume technique, heat flux was calculated from foam and foam-free surfaces as a function of ambient humidity (ranged from 40% to 78%), air–water temperature difference (ranged from −9 °C to 0 °C), and wind speed (variable up to 3 m s−1). Water-surface skin temperature was imaged with a calibrated thermal infrared camera, and near-surface temperature profiles in the air, water, and foam were recorded. Net heat flux from foam surfaces increased with increasing wind speed and was shown to be up to four times greater than a foam-free surface. The fraction of the total heat flux due to the latent heat flux was observed for foam to be 0.75, with this value being relatively constant with wind speed. In contrast, for a foam-free surface the fraction of the total heat flux due to the latent heat flux decreased at higher wind speeds. Temperature profiles through foam are linear and have larger gradients, which increased with wind speed, while foam free surfaces show the expected logarithmic profile and show no variation with temperature. The radiometric surface temperatures show that foam is cooler and more variable than a foam-free surface, and bubble-resolving thermal images show that radiometrically transparent bubble caps and burst bubbles reveal warm foam below the cool surface layer, contributing to the enhanced variability.

Machine learning informed predictor importance measures of environmental parameters in maritime optical turbulence

Prediction of the index of refraction structure constant Cn2 in the low-altitude maritime environment is challenging. To improve predictive models, deeper understanding of the relationships between environmental parameters and optical turbulence is required. To that end, a robust data set of Cn2 as well as numerous meteorological parameters were collected over a period of approximately 15 months along the Chesapeake Bay adjacent to the Severn River in Annapolis, Maryland. The goal was to derive new insights into the physical relationships affecting optical turbulence in the near-maritime environment. Using data-driven machine learning feature selection approaches, the relative importance of 12 distinct, measurable environmental parameters was analyzed and evaluated. Random forest nodal purity analysis was the primary machine learning approach to relative importance determination. The relative feature importance results indicated that air temperature and pressure were important parameters in predicting Cn2 in the maritime environment. In addition, the relative importance findings suggest that the air-water temperature difference, temporal hour weight, and time of year, as measured through seasonality, have strong associations with Cn2 and could be included to improve model prediction accuracy.

Eddy covariance measurements of water vapor and energy flux over a lake in the Badain Jaran Desert, China

Exploring the surface energy exchange between atmosphere and water bodies is essential to gain a quantitative understanding of regional climate change, especially for the lakes in the desert. In this study, measurements of energy flux and water vapor were performed over a lake in the Badain Jaran Desert, China from March 2012 to March 2013. The studied lake had about a 2-month frozen period (December and January) and a 10-month open-water period (February–November). Latent heat flux (LE) and sensible heat flux (Hs) acquired using the eddy covariance technique were argued by measurements of longwave and shortwave radiation. Both fluxes of longwave and shortwave radiation showed seasonal dynamics and daily fluctuations during the study period. The reflected solar radiation was much higher in winter than in other seasons. LE exhibited diurnal and seasonal variations. On a daily scale, LE was low in the morning and peaked in the afternoon. From spring (April) to winter (January), the diurnal amplitude of LE decreased slowly. LE was the dominant heat flux throughout the year and consumed most of the energy from the lake. Generally speaking, LE was mostly affected by changes in the ambient wind speed, while Hs was primarily affected by the product of water-air temperature difference and wind speed. The diurnal LE and Hs were negatively correlated in the open-water period. The variations in Hs and LE over the lake were differed from those on the nearby land surface. The mean evaporation rate on the lake was about 4.0 mm/d over the entire year, and the cumulative annual evaporation rate was 1445 mm/a. The cumulative annual evaporation was 10 times larger than the cumulative annual precipitation. Furthermore, the average evaporation rates over the frozen period and open-water period were approximately 0.6 and 5.0 mm/d, respectively. These results can be used to analyze the water balance and quantify the source of lake water in the Badain Jaran Desert.

A highly efficient solution for thermal compensation of ground-coupled heat pump systems and waste heat recovery of kitchen exhaust air

This paper proposed a highly efficient solution for both thermal compensation of ground-coupled heat pump systems and kitchen exhaust air waste heat recovery (KHR-GCHP) in cold climatic regions. A new analysis and calculation method was developed for non-finned coil heat exchangers of KHR-GCHP systems. Moreover, two indexes were defined to evaluate KHR-GCHP and other relevant systems: the thermal compensation capacity per unit energy consumption (TEC) and the system specific net energy saving (S-SNES). Through comparing and analyzing TRNSYS simulation results and experimental data, it is concluded that the KHR-GCHP system has higher energy efficiency with TEC up to 36.78kWh/kWh, significantly higher than that of normal boiler-GCHP systems (0.90kWh/kWh) or frequently-used hybrid solar-GCHP systems (15.73kWh/kWh). Furthermore, the S-SNES of the KHR system reached 2.37W·h/m3, which was at least 1.86 times as high as common kitchen exhaust air waste heat recovery systems (ranging from 0.85–1.27W·h/m3). Additionally, to facilitate KHR-GCHP’s wide application, a simplified criterion was developed: for both circumstances where air-water temperature difference (Td) was higher than 9.43°C (current project; suitable for the hybrid solar-GCHP system) or higher than 3.99°C (other projects), the KHR-GCHP system demonstrated advantages over other systems.

Dry deposition velocities of submicron aerosols on water surfaces: Laboratory experimental data and modelling approach

This article presents new experimental values of deposition velocities of submicron and hygroscopic aerosols of uranine with the objective to analyze dry deposition mechanisms of submicron aerosols on water surfaces in function of wind, water current and differences between air and water temperatures. A special attention is paid to phoretic effects which have been poorly considered by previous experimental studies. Experiments were performed in a laboratory wind tunnel for different conditions precisely characterized in function of aerosol size distribution, air turbulence, water current, water surface deformations, air and water temperatures and relative humidity. The experimental results do not allow detecting any effects of water current nor water surface deformation. The deposition velocity dimensionless with the friction velocity varies according to a V curve as a function of the air-water temperature difference. The minimum of this curve (≈ 2.0×10−5) is reached when the air-water temperature difference is equal to −7.5°C. To analyse these experimental results, appropriate models of hygroscopicity growth, diffusiophoresis and thermophoresis are implemented in the Slinn resistance model (Slinn & Slinn, 1980). Comparisons between experimental and modelling results confirm that the dry deposition velocity of submicron aerosol over water surface is mainly influenced by the turbulent diffusion and show that the influence of hygroscopicity growth is negligible. Furthermore, it is shown that adding the diffusiophoresis and thermophoresis velocities in the deposition layer transfer coefficient greatly improves the agreement between the experimental deposition velocity and the model. Particularly, these results reveal the important role of the thermophoresis when the air-water temperature difference becomes lower than -3°C.

Spatio-temporal variations of GHG emissions from surface water of Xiangxi River in Three Gorges Reservoir region, China

Damming has exerted far-reaching and profound impacts on ecological functions in river ecosystems. However, the damming effects on greenhouse gases (GHG) emissions from tributaries of reservoirs in subtropical region were largely overlooked. In this study, we assessed the GHG fluxes (CH4, CO2 and N2O) from air-water interface and the related factors in Xiangxi River of Three Gorges Reservoir (TGR) region over one year. The annual average fluxes of CH4, CO2 and N2O from Xiangxi River were 0.13 +/- 0.11 mg m(-2) h(-1), 16.01 +/- 23.21 mg m(-2) h(-1) and 21.57 +/- 12.75 mg m(-2) h(-1), respectively, which were significantly lower than those from tropical rivers or lakes. The GHG fluxes showed strong seasonal dynamics: CH4 was consistently released to the atmosphere and was more in summer than the other seasons. CO2 was absorbed from the atmosphere in spring, but was emitted the most in winter. N2O flux was higher in cold season than in warm season. Additionally, CH4 emission showed obvious spatial variations that decreased gradually from shallow to deep water-level areas. Correlation analysis demonstrated that CH4 emission was negatively correlated with water depth and air-water temperature difference, and positively with air temperature. CO2 flux was negatively related to chlorophyll a (Chl. a) and N2O flux was negatively correlated with dissolved oxygen (DO). The present study will deepen our understanding of Three Gorges Dam (TGD) construction impacts on GHG emissions in the tributaries, and help us to better estimate GHG emissions for TGR. (C) 2015 Elsevier B.V. All rights reserved.

Turbulent Heat Fluxes above a Suburban Reservoir: A Case Study from Germany

Abstract With the objective of quantifying turbulent sensible QH and latent QE heat fluxes above a suburban reservoir, a micrometeorological turbulence measurement campaign using the eddy covariance technique was carried out between June and November 2012 on the Baldeneysee, an artificial lake (2.64 km2 area with up to 4-m depth) located in Germany. The study was conducted at this site because there is little information on this type of lake and the findings are important for the local water management system. Sensible heat fluxes show a mean diurnal course with maximum positive values during the early morning hours (maximum = +31.3 W m−2) and negative values in the afternoon (minimum = −12.3 W m−2). During daytime, average QH/Q* ratios (where Q* is the radiation balance) are, in general, smaller than 0.20. Sensible heat fluxes are especially affected by the product of wind speed and water–air temperature difference, as well as the magnitude of global radiation. In contrast, mean hourly values of QE are, on average, positive over the whole day, with an absolute spread between +175.1 and −0.8 W m−2. During daytime in summer, 74% of the radiation balance is transferred via QE. The measurement values show that during two summer days, 3.72 and 2.84 mm day−1 of water could be evaporated, whereas during two measurement days in autumn, evaporation rates of only 1.36 and 0.94 mm day−1 were registered. The magnitude of QE is mainly affected by four main parameters: the product of wind speed and vapor pressure deficit, followed by the magnitude of global radiation and water temperature.

Comparison between closed static chamber method and thin boundary layer method on monitoring air-water CO<sub>2</sub> diffusion flux

模型估算法与静态箱法是水-气界面气体通量监测的主要方法,因原理不同监测结果通常存在一定差异.目前对引起上述差异的主要环境因素仍不清晰.本研究使用自行设计的静态箱对三峡支流澎溪河水-气界面CO₂通量进行监测,并与同步开展的CO₂通量薄边界层模型估算法结果相比较,探讨该水域引起这两种监测方法结果产生差异的主要环境因素.结果表明,瞬时风速、水汽温差及水深均会对静态箱法及模型估算法的监测结果产生影响.风速越强、水汽温差越大、水深越大,这两种方法监测结果的差异就越小;而水域面积对两种方法的差异没有影响.比较发现,两种方法所获通量数据呈显著正相关,但静态箱法所获通量数据离散性显著高于薄边界层模型估算法.从方法的稳定性角度,在峡谷河道型水库水体温室气体监测中薄边界层模型估算法可能更为适宜.;Closed static chamber method and thin boundary layer method are frequently applied in monitoring air-water gas diffusive fluxes. However, there are differences in results between the methods due to theoretical principles which are still unclear. A type of closed static chamber was applied in Pengxi River, Three Gorges Reservoir to monitor air-water CO₂ flux and compared with the results from thin boundary layer method that simultaneously monitored to discuss the causes for the differences. Results indicated that wind speed, air-water temperature difference, and depth of the water were the impact factors related to the difference between the methods above. It was found that the smaller difference between the methods above was created by the higher wind speed, or the greater air-water temperature difference, or the greater water depth. However, water surface area showed no significant impact on the difference between the methods. Through a comparison of CO₂ fluxes data sets from the methods, it could be noted that there was a significant positive correlation on the monitoring results of the CO₂ fluxes.But monitoring results from the closed static chamber method had higher variance compared to thin boundary layer method.From the viewpoint of the stability of the methods, the thin boundary layer method might be more feasible in the river-channel based reservoir area.

Wind tunnel measurements and numerical simulations of water evaporation in forced convection airflow

The aim of this work is to investigate the relationship between evaporation from heated water surfaces and mean aero thermal properties of a forced airflow. Flows with Reynolds numbers varying between 2475 and 49,503 were considered. Both wind tunnel measurements and numerical simulations were used. The experimental results were obtained in an installation consisting of a low speed wind tunnel and an evaporation tank. The numerical study was performed through a 3D-CFD code. To validate the numerical formulation the predicted results were compared with the experimental measurements for the mass transfer at the free surface of the water tank and with correlations available in the literature. A good agreement was achieved which indicates an interesting capacity of the CFD program to predict the phenomena engaged.The dependence of the rate of evaporation with air velocity, water–air temperature difference and relative humidity is also addressed. The results obtained with both methodologies clearly show that the rate of evaporation is mainly dependent of the airflow velocity. The water–air temperature difference and the relative humidity also have an important effect, but much less than the airflow velocity. For small airflow velocities the rate of water evaporation is also small and presents only a slightly dependence from the temperature difference and the relative humidity.

Annual variation of air-water temperature difference at three Estonian coastal stations

Water and air temperatures during 1980-2010 were compared at three coastal stations of Estonia with the aim to estimate the stability conditions of the lower layers of the atmosphere. It is shown that the difference between the air and water temperatures varies during the year and the annual cycle of this variation is different at different measurement sites and also during different wind directions. Measurements at Sorve demonstrate an unusual situation when the air-sea temperature difference is affected by the wind systems of the Gulf of Riga and Baltic Proper.

Lake Michigan air quality: The 1994–2003 LADCO Aircraft Project (LAP)

The goal of the 1994 to 2003 LADCO Airplane Project (LAP) was to study ozone formation over Lake Michigan so that equitable regional control strategies could be devised. This paper for the first time documents LAP in the peer-reviewed literature. Dye et al. (1995) found that the atmosphere over Lake Michigan is stable in the summer due to the airwater temperature difference, which creates an efficient reaction chamber for ozone formation. They also hypothesize that the southwest winds characteristic of ozone-conducive conditions transport ozone further north over the lake before it crosses the shoreline onto land. This statistical analysis of LAP data support the hypothesis of Dye et al. Below 200 m above the lake, ozone formation is VOC-limited in the morning and becomes NO x limited in the afternoon. Above 200 m, ozone formation is NO x -limited throughout the day. The onshore NO x and VOC diurnal cycles peak during the early morning rush hour and are clearly linked to traffic patterns. Over the lake, VOC and NO y concentrations peak during the mid-morning rather than the early morning, supporting the hypothesis that the land breeze transports VOC and NO y over the lake. The diurnal NO x pattern over Lake Michigan is less clearly defined than the VOC pattern possibly as a result of emissions from five coal-burning power plants located on the Lake Michigan shoreline. Using a “photochemical clock” model, we estimate the climatological average hydroxyl radical concentration over the lake to be (9.43 ± 5.88) × 106 molecule cm −3 near Chicago and (8.43 ± 3.68) × 106 molecule cm −3 near Milwaukee. ► First report of a decade-long airplane study of ozone episodes over Lake Michigan. ► O 3 formation is first VOC-limited then NO x -limited at low altitudes over the lake. ► Average HO • concentration was estimated using a “photochemical clock”.

Long-term energy flux measurements and energy balance over a small boreal lake using eddy covariance technique

[1] Measurements of the energy balance components of a small boreal lake (area 0.041 km2, mean depth 2.5 m) in southern Finland were performed during four open water periods (April–October) in 2005–2008. Turbulent fluxes of sensible and latent heat acquired using the eddy covariance technique were accompanied by net radiation and water heat storage measurements. In April the lake was near isothermal, whereas in May the development of a thermocline was enabled by dark water color and a sheltered location. The thermocline continued to deepen until September down to the depth of 3.5 m and prevented the deeper water from interacting with the atmosphere. The sensible heat flux was governed by the air-water temperature difference and had its minimum in the afternoon (values down to −45 W m−2) and peaked in the early morning (values up to 32 W m−2). The monthly means ranged from −9 W m−2 (April 2005) to 24 W m−2 (July 2008). The diurnal variation of the latent heat flux, controlled by vapor pressure deficit, had an opposite diurnal phase with a maximum in the afternoon (values up to 120 W m−2) and minimum in the morning (values down to ∼2 W m−2). The monthly averages ranged from 4 W m−2 (October 2008) to 77 W m−2 (July 2006). Furthermore, the lake acted as a heat sink until July and August when the maximum heat content was about 230 MJ m−2. The monthly energy balance closure varied from 57% to 112%, and the average closure was 82% (residual 16 W m−2) and 72% (residual 23 W m−2) for 2006 and 2007, respectively.

The effect of humidity and state of water surfaces on deposition of aerosol particles onto a water surface

Deposition processes of particles with dry diameter larger than about 10 μm are dominated by gravitational settling, while molecular diffusion and Brownian motion predominate the deposition processes of particles smaller than 0.1 μm in dry diameter. Many air pollution derived elements exhibit characteristics common to sub-micron particles. The objective of the present study is to examine the effects of meteorological conditions within the turbulent transfer layer on the deposition velocity of particles with dry diameter between 0.1 and 1 μm. It is for these sub-micron particles that particle growth by condensation in the deposition layer, the broken water surface effect and the enhanced transfer process due to atmospheric turbulence in the turbulent transfer layer play important roles in controlling the particle deposition velocity. Results of the present study show that the `dry air’ assumption of Williams’ model is unrealistic. Effects of ambient air relative humidity and water surface temperature cannot be ignored in determining the deposition velocity over a water surface. Neglecting effects of ambient air relative humidity and water surface temperature will result in defining atmospheric stability incorrectly. It is found that the largest effect of air relative humidity on deposition velocity occurs at an air–water temperature difference corresponding to the point of `displaced neutral stability'. For a given wind speed of U=5 m s −1 the additive effects of water surface temperature, T w, changes from 5 to 25°C and ambient air relative humidity variations from 85 to 60%, respectively, lead to a maximum difference in v d of about 20%. For a higher wind speed of 10 m s −1, however, the corresponding change in v d reduces to less than 5%. This is further confirmation that wind speed is one of the strongest variables that governs the magnitude of v d. The present study also found that the broken surface transfer coefficient, k bs, given as a multiple of the smooth surface transfer coefficient, k ss, is physically more meaningful than assigning it a constant value independent of particle size. The method used in this study requires only a single level of atmospheric data coupled with the surface temperature measurement. The present method is applicable for determining deposition velocity not only at the conventional measurement height of 10 m but also at any other heights that are different from the measurement height.

Experimental modelling of the arctic steam fogs

Results of modelling of steam fogs, specific for the Arctic regions, are presented. An experimental setup based on the thermal-vacuum chamber of 100 m 3 in volume (15 m in height, 3 m in diameter) allows one to model and investigate the processes of formation and evolution of the steam fogs in dependence of the water-air temperature difference ( Δ w ), water surface roughness, content of ice nuclei and other factors that are typical for polar regions. The water surface roughness 3–6 times increases the fog optical density. The temperature difference of “water surface-air” is a determinant factor as well, however, the modal size of supercooled droplets is sufficiently conservative to ΔT w variation and the droplet concentration and fog optical density τ are largely determined by the content of nuclei crystallization concentration in the interval N c=0–30 cm −3. If N c concentration increases up to 30–300 cm −3, then, the fog optical density 3–5 times decreases.

Bulk Transfer Coefficients for Heat and Momentum over Leads and Polynyas

Abstract Leads and polynyas are areas of open water surrounded by pack ice. In winter, when the polar oceans have extensive ice covers and the water-air temperature difference is typically 20°–40°C, they allow enormous amounts of sensible and latent heat to escaoe from the ocean to the atmosphere. Parameterizing these fluxes accurately is thus an important part of modeling the growth and decay of sea ice. To develop a unified method for parameterizing the turbulent transfer from open water surrounded by pack ice, we have reanalyzed data reported in the literature on momentum and heat transfer over Arctic leads and polynyas. The neutral stability value of the 10-m drag coefficient, CDN10 = 1.49×10−1, is independent of wind speed and open-water fetch for winds from 1 to 10 m s−1 and fetches from 7 to 500 m. That value is slightly higher than values typical of the open ocean at these wind speed probably because of the form drag over the upwind ice or at the ice edges and because the wave field is still activ...

Whitecaps and the passive remote sensing of the ocean surface

Abstract Whitecap coverage (W), which influences the apparent microwave brightness temperature and short-wave albedo of the sea surface, is not only a strongly non-linear function of the l0m-elevation wind speed (U), but also varies with changes in the stability of the lower atmosphere (i.e. with alterations in the water-air temperature difference AT), and with changes in the surface-sea water temperature (Tw). Thus wind retrieval algorithms to be applied to open ocean data from whitecap-detecting satellite instruments should ideally be of the form, U(W, δT, Tw, d), where d is a measure of the effective wind duration. The wind speed associated with the onset of whitecapping, while also varying with AT and Tw, is typically 3 to 3-5ms-1, not the often quoted 7ms-1

Winds, Upwelling and Fog at Cape Cod Canal, Massachusetts

Abstract New observations confirm the formation of advective fogs over the western approaches to the Cape Cod Canal. The large air-water temperature differences required for the formation of these fogs are shown to occur when the colder Cape Cod Bay waters enter the canal, apparently in response to wind effects in producing upwelling. A simple means of anticipating the formation of these fogs is suggested.